Metal oxide ceramic materials are a class of ceramic materials having a variety of industrial and scientific uses, the most common of which involve separation processes, catalysis, and adsorption. Such ceramic materials can be performed in planar layers, referred to as membranes because of their porosity, or may be sintered into dense monolithic vitrified or solidified glassine materials or blocks. Recently much research has been conducted on making these materials in a more porous form. For example, a method is disclosed in published PCT patent application WO89/00983, by the inventors here, of a method for the creation of polymeric and particulate metal oxide ceramic membranes. The method disclosed in that published patent application for creating particulate ceramic membranes involves the use of relatively large amounts of water in an alcoholic solution, in combination with a mild heating during peptizing, to create appropriate charged particles which can be dewatered and sintered to create a metal oxide porous ceramic membrane.
While membranes are one possible physical form that metal oxide ceramic materials may take, it is not the only one. One of the desirable attributes of many metal oxides formed of transition metals is that the transition metal can have catalytic properties desirable for certain photochemical or electrophotochemical reactions. These materials are also effective adsorbents. Metal oxide porous ceramic materials thus offer a potentially attractive candidate for use as catalytic or adsorbent agents in industrial scale chemical reactions. Metal oxide ceramic materials have great chemical stability, since they are resistant to organic solvents, to chlorine, and to some extremes of pH, to which organic catalytic agents may be susceptible. Ceramic materials are also inherently more stable at high temperatures, and therefore would allow for efficient sterilization of process equipment, another procedure not always possible with organic catalytic agents. Since metal oxide ceramic materials are also entirely inorganic, they are stable and resistant to microbial or biological degradation.
One of the limitations on the previous use of transition metals, and materials made from them, in catalytic or adsorbent processes is the need for the catalytic or adsorbent agents to be in a physical form which allows for a large degree of surface contact between the substrates of the reaction and the catalyst and/or adsorbent. Metal materials are often most readily available in the form of films, solid particles, or crystals, but none of these physical forms has a large degree of surface area which would be desirable for materials used as catalysts or adsorbents. While metallic materials can be coated onto relatively porous substrates, clearly it would be advantageous to have the substrate itself in a physical form which would be both stable and capable of convenient handling, and also which would have great surface area so as to make the catalytic or adsorbent agent available to the substrates for their reaction. One typical form of catalyst or adsorbent used in many industrial scale chemical reactions is a pellet. Such pellets can be loosely packed into beds or reactors. If the pellets are of a sufficient internal porosity, the vapor or liquid pressure drop through a reactor filled with such pellets will be within acceptable bounds. Since such pellets are often used in industrial catalytic or adsorbent processes, it is an advantage of any newly developed catalytic or adsorbent materials that they be capable of manufacture in a form which may readily be accepted by existing industrial applications.
The creation of metal oxide ceramic materials is generally conducted through what is referred to as a sol-gel procedure. In such a procedure, the metal alkoxide is initiated into solution in a solvent, in a reaction vessel in which the solvent is rapidly being stirred. Depending on the process, the solvent may be alcoholic or aqueous. Whichever solvent is used, the metal alkoxide in solution is then hydrolyzed to create metal hydroxide monomers, dimers, polymers and/or particles depending on the quantity of water used. The hydrolyzing metal oxide particles in the solution tend to aggregate and to readily precipitate from solution. The hydrolysis process must therefore be strictly limited by the control of one or more aspects of the process to prevent precipitation of insoluble metal oxide solids from the solution. The insoluble metal oxide particles are thus, in essence, maintained in suspension until they are peptized by the addition of an acid, which causes the particles of the metal oxide to have a greater propensity to remain in suspension, presumably due to opposing charges acquired by the particles during the peptizing process. Such stabilization of the formation of particles has also been accomplished sterically by adding surfactant agents. The stable suspension thus produced, referred to as a sol, is then treated by removal of the solvent therefrom to create a gel or semisolid material. Such gels may be subject to further removal of solvent, and are then sintered or fired to turn the gel from a semisolid into a completely solid, rigid, durable material. In the past such ceramic oxide materials had been formed typically as coatings, or as supported or unsupported membranes, or as monoliths or other solid densified and vitrified objects.